In physics, cryogenics is the study of the production of very low temperatures (below −150° C., −238° F. or 123K) and the behavior of materials at those temperatures. Cryogenic electronics, the operation of electronic devices, circuits, and systems at cryogenic temperatures, has been a valuable technology for decades. Cryogenic electronics can be based on semiconductive devices, on superconductive devices, or on a combination of the two. The investigation and application of semiconductor devices, e.g. diodes and transistors, at low temperatures was underway during the 1960s-1970s and has continued such that semiconductor electronics has since expanded into many areas, based on integrated circuits as well as transistors.
Such semiconductor-based cryogenic electronics can be as simple as a circuit using a single transistor (or diode) or as complex as a system incorporating hundreds of large integrated circuits. It includes both analog and digital systems, spans the frequency spectrum from DC to 100 s of GHz, and ranges in power from microwatts to hundreds of watts. Transistors types include both bipolar and field-effect, using Si, Ge, GaAs, SiGe and III-V semiconductor materials. Cryogenic electronic circuits are used not only in the laboratory, but hundreds have been used “in the field” in practical applications, and several types are available commercially. There are two broad reasons for operating electronics at cryogenic temperatures, first being to improve the performance of the electronics (lower noise, higher speed, increased efficiency, etc.), and second because electronics are required to support a sensor, actuator or other apparatus residing in a cryogenic environment. Some applications may combine both reasons. Related benefits of cryogenic operation may include improved thermal and electrical conductivity, lower operating power, reduction of parasitic losses, diminished chemical and metallurgical degradation, and improved overall reliability.
Historically cryogenic electronics have exploited liquid inert refrigerants such as nitrogen (II) oxide (121K), argon (87K), liquid nitrogen (77K), neon (27K), and helium (4K) to provide the required ambient temperature of operation. In some instances thermoelectric coolers, typically Peltier devices, are employed in conjunction with particular refrigerants, such as liquid nitrogen, to obtain intermediate temperatures. However, in all such instances the electronic circuit must be sealed within a housing thermally isolating the electronics from ambient increasing the size, weight and cost of cryogenic electronics. Accordingly it would be beneficial to provide a means of providing cryogenic cooling of electronics with reduced complexity, reduced ancillary hardware requirements, and cost. In other instances it may be beneficial to provide cryogenic cooling to part of an electronic circuit without cooling the entire circuit.
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect which has been used to attain extremely low temperatures, as well as provide cooling over temperature ranges used in common refrigerators, depending on the design of the system. The effect was first observed by Emil Warburg (1880) and the fundamental principle was suggested by Debye (1926) and Giauque (1927) with the first working magnetic refrigerators were constructed by several groups beginning in 1933 and was the first method developed for cooling below approximately 0.3 K which is attainable using 3He refrigeration. Using magnetic refrigeration temperatures in the micro-Kelvin (μK) to milli-Kelvin (mK) ranges.
The mechanism involves a material in which some aspect of disorder of its constituent particles exists at low temperature, for example at liquid helium temperatures of 4K (4He) or 0.3K (3He). Magnetic dipoles in a crystal of paramagnetic salt, e.g. gadolinium sulfate Gd2(SO4)3.(H2O)8 or cerium magnesium nitrate Ce2Mg3(NO3)12.(H2O)24, have this property of disorder in that the spacing of the energy levels of the magnetic dipoles is small compared with the thermal energy. For paramagnetic salts the active magnetic dipoles are those of the electron shells of the paramagnetic atoms. Under these conditions the dipoles occupy these levels equally, corresponding to being randomly oriented in space. When a magnetic field is applied, these levels become separated sharply; i.e., the corresponding energies are widely different, with the lowest levels occupied by dipoles most closely aligned with the applied field. If the magnetic field is applied while the paramagnetic salt is in contact with the liquid helium heatsink, an isothermal process in which a constant temperature is maintained, many more dipoles will become aligned, with a resultant transfer of thermal energy to the heatsink.
If the magnetic field is decreased after contact with the heatsink has been removed, no heat can flow back in (an adiabatic process), and the paramagnetic salt sample will cool. Such cooling corresponds to the dipoles remaining trapped in the lower energy states, i.e. aligned. Temperatures from 0.3K to as low as 0.0015K have been demonstrated through such paramagnetic salt samples. Much lower temperatures can be attained by an analogous means called adiabatic nuclear demagnetization which relies on ordering (aligning) nuclear dipoles (arising from nuclear spins), which are at least 1,000 times smaller than those of atoms. With this process, temperatures of the ordered nuclei as low as 16 μK (0.000016K) absolute have been demonstrated.
In both techniques, spins are polarized at an initial high magnetic field Bi at an initial temperature Ti, and the magnetic field is then adiabatically swept to a low final field Bf. Owing to the adiabatic nature of the magnetic field sweep, the initial spin temperature Ti is reduced to a final spin temperature Tf=Ti×(Bf/Bi). The electron and phonon degrees of freedom at Ti are subsequently cooled by heat exchange with spins at the lower temperature Tf.
Accordingly such solid state refrigeration may be employed to cool an electronic circuit to provide a cryogenic electronic circuit although the same constraints as described supra in respect of cooling the whole electronic circuit remain. Additionally, it is necessary to provide paramagnetic salts in substrate form allowing the electronic circuit, typically formed upon a silicon substrate, to be mounted upon it. It would be beneficial therefore to remove the requirement for such materials by providing direct magnetic cooling of the electronic circuit substrate or a predetermined portion of the electronic circuit and/or electronic circuit substrate. In general form it would be beneficial to be able to provide direct magnetic cooling of semiconductor materials including but not limited to Si, Ge, GaAs, SiGe, InP, AlGaAs, InGaAsP.
According to embodiments of the invention direct cooling of a semiconductor, e.g. silicon, is achieved through demagnetization of electron spins bound to donor impurity atoms. The entropy of donor-bound electron spins in heavily doped silicon, for example, dominates over both phonon entropy and electron gas entropy at low temperatures and moderate magnetic fields, see for example Lakner et al in “Localized Magnetic Moments in Si:P near the Metal-Insulator Transition” (Phys. Rev. B, Vol. 50, pp 17064-17073) and Wagner et al in “Specific Heat of Si:(P,B) at Low Temperatures” (Phys. Rev. B 55, 4219-4224), thus enabling effective cooling by demagnetization. Direct magnetic cooling of silicon substrates as outlined above being of practical importance for silicon device applications in cryogenic electronics requiring low temperatures, including, but not limited to silicon based quantum computing, see for example Maune et al in “Coherent Singlet-Triplet Oscillations in a Silicon-Based Double Quantum Dot” (Nature, Vol. 481, pp 344-347), and transition edge single photon detectors, see for example Lita et al in “Counting Near-Infrared Single-Photons with 95% Efficiency” (Opt. Exp., Vol. 16, pp 3032-3040).
Beneficially active electronic devices can be integrated monolithically atop a semiconductor substrate exhibiting magnetic refrigeration within the whole substrate or predetermined regions of the substrate. Alternatively, active electronic devices may be formed with semiconductor layers integral within them that exhibit magnetic cooling.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.